Crystal Structure and Microwave Dielectric Properties of LiRE₉(SiO₄)₆O₂ Ceramics (RE = La,Pr,Nd,Sm,Eu,Gd, and Er)

Abstract

The crystal structure and microwave dielectric properties of apatite‐type LiRE9(SiO4)6O2 ceramics (RE = La, Pr, Nd, Sm, Eu, Gd, and Er) have been investigated. The densification of lithium apatites has been greatly improved with the addition of 1 wt% LiF. Selected area electron diffraction and X‐ray diffraction (XRD) Rietveld analysis confirm that these compounds belong to the P63/m (No. 176) space group with hexagonal crystal symmetry. The porosity‐corrected relative permittivity was found to decrease with decreasing ionic polarizability of RE3+ ions. Relationships between the structural parameters and microwave dielectric properties have been examined. The observed variation in the quality factor of LiRE9(SiO4)6O2 + 1 wt% LiF ceramics (RE = La, Pr, and Nd) was correlated with average cation covalency (%). The temperature coefficient of resonant frequency was found to depend on the bond valence sum of cations. LiEr9(SiO4)6O2 + 1 wt% LiF ceramics showed good microwave dielectric properties with εr = 12.8, Qu × f = 13000 GHz and τf = +17 ppm/°C. All the compositions showed low coefficient of thermal expansion with thermal conductivity in the range 1.3–2.8 W (m K)−1.

Effect of TiO2 Doping on the Structure and Microwave Dielectric Properties of Cordierite Ceramics

“…Moreover, in silicate structures, Al 3+ ions would like to occupy the Si 4+ ionic sites to compose the [AlO 4 ] tetrahedrons with a negative charge vacancy. Some silicates such as Cristobalite (SiO 2 ), Forsterite (Mg 2 SiO 4 ), Pyroxene (CaMgSi 2 O 6 ), Sr‐Gehlenite (Sr 2 Al 2 SiO 7 ), Anorthite (CaAl 2 Si 2 O 8 ), Sr‐Feldspar (SrAl 2 Si 2 O 8 ), Willemite (Zn 2 SiO 4 ), Cordierite (Mg 2 Al 4 Si 5 O 18 ), Apatite‐type [LiRe 9 (SiO 4 ) 6 O 2 ] (Re = rare earth elements), Mullite (Al 6 Si 2 O 13 ), etc., are the alternative candidates for microwave and millimeter‐wave dielectric materials, which have been fabricated and reported during the recent years.…”

Effect of Ca2+ Substitution on the Structure, Microstructure, and Microwave Dielectric Properties of Sr2Al2SiO7 Ceramic

“…Forsterite ceramics have been reported with high Q u × f and low ε r at microwave frequencies, suggesting their possible use as microwave substrates as well as millimeter‐wave dielectric resonators . Several other silicates have been also reported for substrate applications . As the dielectric properties are affected by intrinsic structural parameters, a proper investigation of crystal structure is also desirable for controlling the overall performance of the substrate.…”

“…Several other silicates have been also reported for substrate applications . As the dielectric properties are affected by intrinsic structural parameters, a proper investigation of crystal structure is also desirable for controlling the overall performance of the substrate.…”

Microwave dielectric properties of mineral sillimanite obtained by conventional and cold sintering process

“…[1]. New microwave substrate materials with low relative permittivity and high performance are desired for high speed communication devices [2]. For microwave substrate applications the dielectric materials should possess low dielectric constant (εr), low loss(tanδ), low temperature coefficient of permittivity (τε), high thermal conductivity and low coefficient of thermal expansion [3].…”